Interface engineering-inspired electron regulation in Pt/Pd hetero-metallene for methanol-assisted hydrogen evolution

The small molecule oxidation reaction instead of oxygen evolution reaction coupled with hydrogen evolution reaction can greatly reduce the reaction overpotential of electrochemical water splitting, which is a very efficient and energy-saving hydrogen evolution strategy. Herein, we report an interface engineering constructed two-dimensional ultrathin curled Pt/Pd hetero-metallene for efficient electrocatalytic hydrogen evolution assisted by methanol. The thin-sheet structure of Pt/Pd hetero-metallene provides a large specific surface area and exposes numerous surface atoms that could act as reactive sites, thus accelerating the reaction mass transfer process. More importantly, the constructed Pt/Pd hetero-metallene possesses abundant Pt/Pd heterointerface, which can maximize the strong metal-metal interaction and increase the utilization of metal atoms, thereby optimizing the adsorption and activation of reactants during the reaction. Pt/Pd hetero-metallene can produce hydrogen stably and efficiently in 1 M KOH + 1 M CH 3 OH, and the voltage only needs 0.83 V at @100 mA cm -2 when used in electrocatalytic hydrogen evolution, which is much lower than the voltage required for the traditional electrochemical water splitting process (1.94 V). This work not only provides a powerful approach to rational design and construction of hetero-metallene through interface engineering, but also builds a bridge between hetero-metallene and methanol-assisted hydrogen evolution.

The small molecule oxidation reaction instead of oxygen evolution reaction coupled with hydrogen evolution reaction can greatly reduce the reaction overpotential of electrochemical water splitting, which is a very efficient and energy-saving hydrogen evolution strategy. Herein, we report an interface engineering constructed twodimensional ultrathin curled Pt/Pd hetero-metallene for efficient electrocatalytic hydrogen evolution assisted by methanol. The thin-sheet structure of Pt/Pd heterometallene provides a large specific surface area and exposes numerous surface atoms that could act as reactive sites, thus accelerating the reaction mass transfer process. More importantly, the constructed Pt/Pd hetero-metallene possesses abundant Pt/Pd heterointerface, which can maximize the strong metal-metal interaction and increase the utilization of metal atoms, thereby optimizing the adsorption and activation of reactants during the reaction. Pt/Pd hetero-metallene can produce hydrogen stably and efficiently in 1 M KOH + 1 M CH 3 OH, and the voltage only needs 0.83 V at @100 mA cm -2 when used in electrocatalytic hydrogen evolution, which is much lower than the voltage required for the traditional electrochemical water splitting process (1.94 V). This work not only provides a powerful approach to rational design and construction of hetero-metallene through interface engineering, but also builds a bridge between hetero-metallene and methanol-assisted hydrogen evolution.
anodic oxygen (O 2 ) evolution reaction (OER) and the cathodic H 2 evolution reaction (HER). [14][15][16] It is worth noting that the traditional electrochemical water splitting OER process requires high overpotentials and generates low value-added O 2 (which may be explosive when mixed with H 2 ). 17,18 Despite the development of many efficient catalysts to attempt to solve this problem, the desired efficiency has not been achieved.
Recently, the oxidation-assisted water electrolysis of small molecules (alcohols, hydrazine, glycerol, etc.) has attracted much attention. [19][20][21] Compared with traditional water electrolysis, it can not only greatly reduce the overpotential, but also may produce high value-added products. 22,23 Among them, methanol (CH 3 OH) is regarded to be an ideal small molecule for oxidation-assisted water electrolysis due to its advantages of easy storage, low price, and low toxicity. [24][25][26] Therefore, replacing OER with methanol oxidation reaction (MOR) to assist water electrolysis is a very efficient and energy-saving strategy. The key to driving methanol-assisted H 2 evolution (HER at the cathode and MOR at the anode) lies in the controllable preparation of high catalytic activity catalysis. It is well known that Pt is the most effective electrocatalyst for HER and MOR. [27][28][29] However, the high price and scarcity of Pt severely limit its widespread commercial application. 30,31 It is crucial to design and construct efficient Pt-based catalysts to enhance the catalytic activity as well as to improve the utilization of Pt in the catalyst.
Pd has a similar electronic structure to Pt, so it has similar physicochemical characteristics to Pt, and it is feasible to use Pd to replace most of the Pt in the catalyst. [32][33][34] The controllable construction of morphology is one of the effective strategies to improve the catalytic performance of Pd-based nanomaterials. In which, two-dimensional (2D) nanomaterials exhibit unique strain effects, interfacial effects and ligand effects that can greatly improve the catalytic performance. [35][36][37][38][39] It is noteworthy that defect-rich metallene, which has a similar structure to graphene, has been efficiently used in electrocatalytic reactions due to its high electrical conductivity, high specific surface area and fast mass transfer. [40][41][42][43] For example, Guo and co-authors have prepared high-quality PdMo bimetallene with sub-nanometer thickness by a simple one-pot method, and the quantum size effect exhibited by the catalyst optimized the electronic structure between metals, resulting in excellent electrocatalytic performance. 44 In addition, the construction of bimetallic catalysts to change the electronic configuration of the catalyst surface through ligand effects and induce facet effect to increase the catalyst active area is also an effective way to improve the catalytic performance. [45][46][47] Among them, the hetero-interface is constructed by depositing ultra-small metal nanoparticles on the metal interface, and

Accepted Article
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record.
the strong interaction between metal and metal is generated to amplify the interface effect, which could greatly improve the utilization rate of metal atoms. [48][49][50] Therefore, the development of ultrathin metallene with Pt/Pd heterointerface is expected to efficiently stimulate the methanol-assisted H 2 evolution. Herein, we construct ultrathin and highly wrinkled 2D Pt/Pd hetero-metallene through interface engineering. On the one hand, the unique 2D ultrathin nanosheetlike structure of Pt/Pd hetero-metallene has a very large specific surface area, thus exposing abundant catalytic active centers. On the other hand, Pt/Pd hetero-metallene is composed of abundant uniform ultra-small Pt nanoparticles anchored on the surface of Pd metallene. The formed abundant Pt/Pd heterointerface could spontaneously generate a unique built-in electric field to optimize the electronic configuration at the interface, thereby enhancing the adsorption and activation of reaction intermediates. The constructed Pt/Pd hetero-metallene exhibit excellent HER catalytic performance in both 1 M KOH and 1 M CH 3 OH + 1 M KOH. In addition, when Pt/Pd heterometallene is applied to methanol-assisted H 2 evolution, the voltage could be reduced to 0.83 V at @100 mA cm -2 , which is significantly better than the traditional electrolysis of water for H 2 evolution (1.94 V). This study provides a novel strategy for the design and preparation of hetero-metallene and offers insights into how to efficient and energy-saving produce H 2 .

Accepted Article
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record.

Accepted Article
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record.

Accepted Article
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record.  The crystal structure of Pt/Pd hetero-metallene was studied by X-ray diffraction (XRD). As shown in Figure. 3a, the XRD pattern shows four distinct diffraction peaks of the metal fcc structure, confirming the polycrystalline structure of Pt/Pd heterometallene. In addition, compared with Pd metallene, it is found that the XRD diffraction peak width increases after Pt NPs deposition, and the peak position does not shift significantly. The elemental composition and valence distribution of Pd  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58 59 60

Accepted Article
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record.  Figures. 3e and S5). The charge transfer forms the generation of built-in electric field at the Pt/Pd heterointerface, which modulates the electronic configuration between the Pt and Pd atoms, and further promotes the adsorption and activation of the reactants. The charge density difference plots further indicate that the positive charge is accumulated in the Pt/Pd heterointerface, which is more likely to accept the lone pair electrons of the reactant molecules, thereby facilitating the adsorption of reactant molecules ( Figures. 3f-3i).

Accepted Article
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record.  hetero-metallene was obtained as 66.3 m 2 g -1 , which is much larger than that of pure Pd metallene (56.8 m 2 g -1 ) and Pt/C (45.3 m 2 g -1 ) (Figure. S6). The results indicate that Pt/Pd hetero-metallene with ultrathin nanosheet structures and abundant Pt/Pd heterointerfaces can provide more catalytically active sites. The MOR performance testing of Pt/Pd hetero-metallene was performed at a scan rate of 50 mV s -1 in N 2 saturated 1 M CH 3 OH + 1 M KOH. The current normalized by the noble metal mass (NM) represents the mass activity (MA). As shown in Figure. 4a, the MA of Pt/Pd hetero-metallene is 1.82 mA μg -1 NM , which is higher than that of Pd metallene (1.11 mA μg -1 NM ) and Pt/C (0.48 mA μg -1 NM ). The current normalized by ECSA represents specific activity (SA). The Pt/Pd hetero-metallene has the highest specific activity (2.76 mA cm -2 ), which is much higher than that of Pd metallene (1.97 mA cm -2 ) and Pt/C (1.05 mA cm -2 ) ( Figures. 4b and 4c). Furthermore, the MOR activity of Pt/Pd hetero-metallene was superior to that of the recently reported catalysts (Table S1). Pt/Pd hetero-metallene has lower onset potential compared to Pd metallene and Pt/C, suggesting that the formation of Pt/Pd heterointerfaces optimizes the intermetallic electronic configuration and results in better reaction kinetic activity (Figure. S7). In  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58 59 60

Accepted Article
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record.  The HER performance of Pt/Pd hetero-metallene was then investigated in 1 M KOH, where the current density was normalized to the geometric area of the glassy carbon electrode. As shown in the linear sweep voltammetry (LSV) curves of each sample, to reach a current density of @-10 mA cm -2 , the Pt/Pd hetero-metallene requires an overpotential of 35 mV, which is lower than that of the Pd metallene (283 mV) and Pt/C (46 mV) ( Figures. 5a and 5b). Notably, the Pt/Pd hetero-metallene also exhibited superior HER activity at 1 M KOH compared with recently reported catalysts (Table S2). The ultrathin nanosheet structure of Pt/Pd hetero-metallene provides sufficient active sites, while the abundant Pt/Pd heterointerface enhances the adsorption and dissociation of H 2 O. In addition, the HER reaction kinetics of each  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58 59 60

Accepted Article
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. sample was further investigated by the Tafel slope plot. The Pt/Pd hetero-metallene (49 mV dec -1 ) has the lowest Tafel slope value compared to the Pd metallene (279 mV dec -1 ) and Pt/C (62 mV dec -1 ), indicating that the Pt/Pd hetero-metallene has excellent HER kinetics ( Figure. 5c). For the next step of CH 3 OH reforming for H 2 evolution, the effect of CH 3 OH on the HER performance was investigated. In 1 M CH 3 OH + 1 M KOH, Pt/Pd hetero-metallene exhibited HER activity comparable to that exhibited in 1 M KOH (Figure. 5d). Next, the charge transfer rate of the catalyst was investigated by electrochemical impedance spectroscopy. Compared with Pd metallene, Pt/Pd hetero-metallene exhibit smaller charge transfer resistance in both HER and MOR ( Figure. S8). This suggests that Pt/Pd hetero-metallene has a superior charge transfer rate, which can be attributed to the built-in electric field spontaneously constructed by the abundant heterointerfaces, enriching a large amount of positive charges in the interface region. The HER stability of Pt/Pd hetero-metallene was further investigated by CV cycle test and chronopotentiometry test. After 5000 CV cycles, the polarization curves of Pt/Pd hetero-metallene showed almost no decay compared with the initial curves ( Figure. 5e). Meanwhile, the overpotential hardly decreased when tested at a current density of @-10 mA cm -2 for 15 h, indicating the excellent HER stability of Pt/Pd hetero-metallene ( Figure. 5f).   1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58 59 60

Accepted Article
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Based on the excellent activity and stability of Pt/Pd hetero-metallene in MOR and HER, Pt/Pd hetero-metallene ink drops were dropped onto the surface of carbon paper as the electrodes for anodic MOR and cathodic HER (Pt/Pd hetero-metallene/CP || Pt/Pd hetero-metallene/CP) to construct a two-electrode system (Figure. 6a). The performance of Pt/Pd hetero-metallene for CH 3 OH assisted H 2 evolution was evaluated by comparing with (Pd metallene/CP || Pd metallene/CP) and (Pt/C/CP || RuO 2 /CP) two-electrode systems. The LSV of Pt/Pd hetero-metallene/CP at 1 M CH 3 OH + 1 M KOH and 1 M KOH as shown in Figure. 6b. When reaching a current density of 100 mA cm -2 , with the assistance of 1 M CH 3 OH was 0.83 V lower than without the addition of CH 3 OH. This fully proves that the introduction of smallmolecule CH 3 OH can greatly reduce the overpotential required for H 2 evolution, thus achieving energy-saving and efficient H 2 evolution. Further at a current density of 100 mA cm -2 , the obtained potential is similarly much smaller than that of Pd metallene/CP || Pd metallene/CP and Pt/C/CP || RuO 2 /CP ( Figure. 6c). In addition, the voltage required for Pt/Pd hetero-metallene in CH 3 OH-assisted H 2 evolution is also better than that of the reported small molecule-assisted H 2 evolution catalysts (Table S3). The LSV curves of Pt/Pd hetero-metallene in 1 M KOH with different concentrations of CH 3 OH were significantly better than those obtained in 1 M KOH without CH 3 OH (Figure. 6d). The results show that the concentration of CH 3 OH has little effect on the catalytic activity ( Figure. 6e). Stability is an important indicator for evaluating catalysts, and the current density changes are negligible by long-time chronopotentiometry (V-t) measurement in 1 M CH 3 OH + 1 M KOH for 25 h, indicating that Pt/Pd hetero-metallene have excellent stability in the CH 3 OH assisted H 2 evolution (Figure. 6f). In addition, the morphology and structure of Pt/Pd heterometallene did not significantly change after a long-time V-t measurement ( Figure. S9). In order to better analyze the product distribution of MOR, nuclear magnetic resonance (NMR) was performed on the electrolyte solution before and after the test. According to the 1H and 1C spectra, obvious peaks of carbonate and formate appeared after the test, thus indicating that the products of MOR are carbonate and formate (Figure. S10).  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58 59 60

Accepted Article
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. When using small-molecule CH 3 OH-assisted H 2 evolution, the voltage required for Pt/Pd hetero-metallene is much lower than that required for conventional water electrolysis, indicating that small molecule assisted H 2 evolution is indeed an energysaving strategy. Benefiting from the advantages of structural engineering and surfaceinterface engineering, Pt/Pd hetero-metallene exhibit excellent performance in CH 3 OH-assisted H 2 evolution. The ultrathin nanosheet structure of Pt/Pd heterometallene can expose more active atoms, thereby facilitating charge transfer. [54][55][56][57] In addition, Pt/Pd hetero-metallene has a large number of Pt/Pd heterointerfaces, and the built-in electric field constructed spontaneously at the interface can adjust the electronic structure of Pt and Pd atoms, thereby enhancing the adsorption and activation of reactant molecules. 58,59

Conclusions
In conclusion, ultrasmall Pt NPs are uniformly anchored on Pd metallene by interface engineering, resulting in the formation of abundant Pt/Pd heterointerfaces. The Pt/Pd hetero-metallene has an ultrathin wrinkled nanosheet structure, exhibiting an extremely large specific surface area, exposing more catalytically active sites. In addition, the constructed Pt/Pd heterointerface modulates the electronic configuration between metal atoms, which could amplify the metal/metal interface effect interaction, thereby optimizing the adsorption and activation of reaction intermediates to further accelerate the reaction kinetics. The Pt/Pd hetero-metallene in a two-electrode system for CH 3 OH-assisted H 2 evolution can exhibit excellent H 2 evolution activity and stability at a low voltage of 0.83 V at @100 mA cm -2 , which is far superior to traditional electrochemical water splitting. This study not only provides an effective strategy for the preparation of high-efficiency small molecule-assisted H 2 evolution electrocatalysts, but also provides a reference for interface engineering to construct hetero-metallene.

Accepted Article
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Additional results for material characterizations and electrochemical measurements, and Table S1-S3 comparisons of the performance.

Accepted Article
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record.

Accepted Article
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record.